One type of a conventional optical coupler is of a bulk type which comprises first and second rod lenses, and a wavelength separating filter sandwiched by the first and second rod lenses.
In operation, lights having wavelengths .lambda..sub.1 and .lambda..sub.2 are incident to the first rod lens and propagated therethrough to be supplied to the wavelength separating filter, so that the light of the wavelength .lambda..sub.1 and the light of the wavelength .lambda..sub.2 are separated by the wavelength separating filter. The light of the wavelength .lambda..sub.1 is then passed through the wavelength separating filter to be propagated through the second rod lens, while the light of the wavelength .lambda..sub.2 is then reflected by the wavelength separating filter, so that the light of the wavelength .lambda..sub.2 is propagated in a direction opposite to the incident direction through the first rod lens. Consequently, the light of the wavelength .lambda..sub.1 is obtained from the second rod lens, while the light of the wavelength .lambda..sub.2 is obtained from the first rod lens.
Another type of a conventional optical coupler is an optical fiber fusing type which comprises a pair of optical fibers, cladding layers of which are fused side by side, such that cores of the optical fibers are adjacent to each other to provide an evanescent field coupling.
In operation, wavelength division multiplexing lights having wavelengths .lambda..sub.1 and .lambda..sub.2 are incident to one of the optical fibers and propagated therethrough to be supplied to a fused portion, so that the light of the wavelength .lambda..sub.2 is almost all coupled in the fused portion to the other optical fiber. Consequently, the light of the wavelength .lambda..sub.1 is obtained at an output end from one of the optical fibers, while the light of the wavelength .lambda..sub.2 is obtained at an output end from the other optical fiber.
However, the bulk type optical coupler has disadvantages in that the number of parts including terminals and connectors (to be explained later) are large which increases the number of assembling steps, and makes it difficult to decrease the insertion loss; although the advantage is the property of isolating the separated lights of the wavelength .lambda..sub.1 and .lambda..sub.2.
Similarily, the fiber fusing type optical coupler has a disadvantage in that the isolation between the separated lights of the wavelengths .lambda..sub.1 and .lambda..sub.2 is not large as expected, although it has advantages in that the number of parts is small to decrease the number of assembling steps, and the insertion loss is small.